The present invention relates to a manufacturing method of substrate means for optical recording disk comprising a disk-like substrate and a transfer layer having grooves formed on a surface of the substrate, and also relates to an apparatus for embodying the manufacturing method.
Generally, in an optical recording disk, tracking grooves are provided on a surface of a substrate in the form of concentric circles or spiral. As a method of providing such grooves, various methods are known. Among these, there has been proposed a method wherein the substrate and the grooves are integrally formed by utilizing a plastic injection molding technique. There has also been proposed a photo-polymerization method, so called, 2P method wherein a transfer layer having the grooves shall be provided on the substrate.
In these methods, the latter 2P method is effected in accordance with the following processes shown in FIGS. 7 to 10. That is, an ultraviolet setting resin solution 112a is coated onto either a surface of a groove forming original board 130 (hereinafter referred to as a "stamper 130") or a light transmissive substrate 111 which is formed of an optical glass or a transparent resin in the form of concentric circle with respect to a central axis (see FIG. 7). Then, the stamper 130 and the substrate 111 are press-contacted to each other thereby to spread over the coated ultraviolet setting resin solution. Thereafter, the ultraviolet setting resin solution 112a is set (or hardened) by irradiating the ultraviolet rays (UV) through the substrate 111, thereby to form a transfer layer 112 (see FIG. 8). When the transfer layer which is integrally formed with the substrate is released from the stamper 130, a groove pattern corresponding to a configuration of the stamper 130 is transferred and reproduced on a surface of the hardened transfer layer 112 (see FIGS. 9 and 10). Subsequently, a recording layer is formed on the surface of the transfer layer 112, and further, another substrate such as a protection plate is provided on the recording layer through an adhesive layer, whereby a conventional optical recording disk is manufactured.
In the transferring and reproducing process for forming the grooves of the conventional optical recording disk, however, the following problems have been raised.
That is, for example, a glass substrate to be used as a substrate for the optical recording disk does not originally have a completely flat surface, but necessarily has a camber of in some degree caused by various forming strains. Therefore, even though the transfer layer formed by utilizing the 2P method is formed on such a glass substrate, the thickness of the transfer layer will not become uniform. The non-uniformity of thickness of the transfer layer will cause a serious problem. That is, because an optical system of the optical recording disk has been designed in consideration of an aberration due to the thickness of the substrate so as to increase an aperture number (NA) of a lens, the non-uniformity of thickness of the transfer layer will have a great influence on the aberration.